The lentiviruses all encode for a Vif protein that has been shown to interact with members of the apolipoprotein mRNA-editing, catalytic polypeptide-like 3 (APOBEC3) superfamily of proteins. The APOBEC3 proteins are cytidine deaminases that are thought to play an important role in innate anti-viral immunity. Humans and macaques both encode for seven APOBEC3 genes (A3A, A3B, A3C, A3D, A3F, A3G, and A3H). The APOBEC3 proteins can be broadly divided into the single deaminase domain (A3A, A3C, and A3H) and double deaminase domain proteins (A3B, A3D, A3F, and A3G). Human A3A (hA3A) does not restrict the replication of HIV-1 vif in 293 cells or HeLa cells but has recently been shown to inhibit replication of HIV vif in macrophage-derived macrophages (MDM). In contrast, rhA3A from rhesus macaques is capable of restricting both HIV-1 and SHIV (expressing the SIV Vif). We recently have assessed the ability of other non-human primate A3A proteins to restrict HIV-1 and HIV-1 vif. We present preliminary data that the A3A protein from the Old World monkey, Colobus guereza (the mantled guereza, colA3A), not only inhibits the replication of HIV-1 and HIV-1 vif but also inhibits virus production in producer cells. Our preliminary studies indicate that the colA3A inhibits by a novel post-entry mechanism and, but not after, proviral integration. In Specific Aim 1, we propose to determine the mechanism though which colA3A inhibits HIV-1 replication and assess the molecular determinants involved. In Specific Aim 2, we propose to determine whether expression of colA3A in human CD4+ T cells and macrophages can restrict HIV-1 in cells it naturally infects. The proposed studies will provide mechanistic insight into this novel mechanism of A3 protein inhibition of HIV-1, which could serve as a new anti-viral strategy against HIV-1.